The Global Positioning System (GPS) is a satellite based location and time transfer system developed by the United States government and available free of charge to all users. Other satellite positioning systems (SPS) have also been or are being developed, including the Glonass satellite system in Russia and the Galileo system in Europe.
The location of an SPS receiver is based upon a one-way ranging between the SPS receiver and several satellites, which transmit signals having the times-of-transmission and orbital parameters for their respective time variable locations-in-space. An SPS receiver acquires satellite signals by correlating internal replica signals to carrier frequencies and distinguishable codes for each of several in-view satellites. When satellite signals have been acquired, the SPS receiver uses time and the orbital parameter information from the acquired satellites for measuring ranges to the satellites, preferably four or more satellites. These measured ranges are called pseudoranges because they include a term caused by a time error of the SPS receiver clock.
The SPS satellite pseudoranges are measured by determining phase offsets between pseudorandom (PRN) codes of the received satellite signals and the internal replica PRN codes referenced to the SPS receiver clock. Some SPS receivers measure and integrate the carrier phases of the satellite signals in order to reduce noise on the measured phase offsets. The SPS receiver then determines an SPS-based time by monitoring the SPS signals until a TOW field is decoded. The SPS-based time is used to determine the times that the phase offsets were measured. The measurement times are then used with ephemeris data received from the satellites for calculating instantaneous locations-in-space of several satellites and for linearizing location equations relating the calculated locations-in-space to the measured pseudoranges. Having four or more linearized location equations for four or more satellites, respectively, SPS receivers can resolve their 3-dimensional geographical location and correct the time error in their internal clocks.
It is known generally to use almanac and ephemeris information stored on SPS receivers to speed the acquisition of satellites. The almanac data contains coefficients to Kepler's equations of satellite motion and is useful for computing which satellites are visible at a particular time. The almanac data may also be used for computing satellite location and velocity vectors, from which satellite Doppler estimates may be computed for aiding signal acquisition. The almanac data provides low-resolution satellite position accuracy, which is typically no better than about 1 kilometer when fresh. Almanac data however contains a relatively small number of bytes, approximately 1200 bytes for 32 satellites, and almanac data is useful for 6 months to 1 year depending on whether satellites are re-positioned or new satellites have been added or removed from the constellation. In the GPS constellation, each satellite broadcasts almanac data, which is updated every few days, for all GPS satellites on a twelve and one-half minute cycle.
Ephemeris data is similar to almanac data but provides far more accurate satellite position information, which is accurate to within several meters if the ephemeris data is not more than a few hours old. The accuracy of satellite position information derived from ephemeris data degrades with time. SPS receivers typically use ephemeris data for computing precise satellite locations, which may be used for position computation when combined with SPS receiver measured pseudorange information. A GPS constellation ephemeris data set for one satellite is approximately 72 bytes of data, and thus ephemeris data for all 32 GPS satellites requires about 2304 bytes of data storage space. In the GPS constellation, each satellite broadcasts its own ephemeris data every thirty-seconds. An SPS receiver must acquire a satellite in order to obtain its ephemeris data.
In a typical GPS receiver, for example, in GPS enabled cellular communications and stand-alone navigation devices, the time to acquire new almanac data directly from a satellite requires more than twelve and a half minutes (12.5 minutes). Operating GPS receivers for the relatively long period required to obtain almanac data directly from a satellite draws substantially charge from the battery, which is undesirable in many applications including GPS enabled cellular telephones. The time required to obtain ephemeris data in this manner is comparatively small, at approximately thirty seconds (30 sec.).
It is known to provide almanac information to GPS enabled radio communications devices in an over-the-air radio message, as disclosed, for example, in U.S. Pat. No. 6,064,336 entitled “GPS Receiver Utilizing A Communication Link”, among other patents and publications. In some instances, however, it is undesirable to use almanac information or to obtain it in an over-the-air message.
It is also known to provide ephemeris information to GPS enabled radio communications devices in an over-the-air radio message, as performed, for example, by the Motorola Eagle GPS receivers. In prior art FIG. 1, GPS satellite ephemeris and almanac information 10 is transmitted from a cellular communications network base-station 12 to a wireless subscriber device 14 using an over-the-air communications protocol. Wireless subscriber device 14 contains a GPS receiver 16 with an antenna, a cellular transceiver 20, and two databases, stored in memory, to store ephemeris data 22 and almanac data 24. The GPS receiver 16 can acquire both almanac and ephemeris data directly from GPS satellites via antenna 18 and store them into the almanac database 24 and ephemeris database 22. In addition, the cellular transceiver 20 can acquire fresh almanac and ephemeris data 10 from the cellular network via over-the-air messages.
Transmitting satellite almanac and ephemeris data over a communications link however requires costly network infrastructure. Additionally, relatively long data strings are required for the transmission of ephemeris and almanac data, and the management of requesting and storing the data derived from over-the-air messages is cumbersome. Other GPS receiver applications, including vehicle navigation, do not include a radio, which could be used for receiving over-the-air assistance messages. For these and other reasons, in at least some applications, it is undesirable to obtain almanac data from over-the-air assistance messages.
U.S. Pat. No. 6,437,735 entitled “Position Detection System Integrated into Mobile Terminal” discloses receiving ephemeris data at a mobile GPS receiver either directly from GPS satellites or from a wireless communications network, and transforms the ephemeris data to almanac information by scaling and masking ephemeris parameters to form corresponding almanac parameters, which are stored on the GPS receiver for positioning determination. Almanac data derived in this manner is believed have substantial errors, for example, accumulated error in the along-track direction, which will likely produce unacceptable results over very long time periods.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below.